In the paint industry, various attempts have been made to decrease the solvent ratio in a coating composition thereby increasing the solid contents which remain on the substrate as a coating. Particular attention has been directed, inter alia, to high solids containing coating compositions. In order to obtain such high solids paint, it is essential to use a resin with a lower molecular weight. However, in the preparation of such resins, there are many problems still awaiting satisfactory solutions.
That is, a low molecular weight resin may be prepared by using an increased amount of a chain transfer agent, such as mercaptans, but the thus obtained resin can only give a coating with an inferior water resistance and an insufficient weather resistance.
Alternatively, such a resin may be prepared by using a higher concentration of a polymerization initiator or by adopting a higher reaction temperature. However, in those cases, there are problems such that the employable reaction solvents are limited and further a particular reactor is required therefor. Thus, a fully satisfactory method for the preparation of a lower molecular weight resin has not yet been found.
There has also been known a technique wherein 2,4-diphenyl-4-methyl-1-pentene is used as a chain transfer agent in the preparation of homo- or copolymer of vinyl ester with a low polymerization degree, as stated in, for example, Japanese Patent Application Kokai No. (unexamined publication No.) 118489/78. Since this type of chain transfer agent does not include any mercapto groups, it is able to obviate the drawbacks inherent in the methods of using a conventional mercaptan chain transfer agent.
The abovesaid technique, however, concerns a polymerization of monomers composed mainly of vinyl esters and even if other copolymerizable monomers, such as acrylic acid and the like, coexist within the system, the weight ratio of such monomers are preferably limited to a lower level of less than 20 mole %.
Moreover, according to the above techniques, the amount of 2,4-diphenyl-4-methyl-1-pentene is only limited to a very low level of 0.001 to 1 weight % of the total monomers used. It is also stated that with the increase in the amount of chain transfer agent over the said range, there is a trend, such that vinyl ester polymerization speed decreases and the polymerization degree is extremely lowered.
The present inventors, having endeavored to obtain an acrylic resin having a lower molecular weight, e.g., a number average molecular weight 500 to 5000, which may be useful as a resinous vehicle in a high solids content paint, and have found that an ideal acrylic resin composition for the intended object can be obtained by using a particular and substantive amount of 2,4-diphenyl-4-methyl-1-pentene for the polymerization of monomers comprising alkyl(meth)acrylate and hydroxy bearing ethylenically unsaturated compounds, and on the basis of said finding, have come to the present invention.
According to the invention, there is provided an acrylic resin composition for use in a high solids paints consisting essentially of the acrylic resin having a number average molecular weight of 500 to 5000 and being prepared by subjecting a mixture of
(A) alkyl(meth)acrylate: 30 to 88 weight % PA1 (B) 2,4-diphenyl-4-methyl-1-pentene: 2 to 60 weight % PA1 (C) hydroxy bearing ethylenically unsaturated monomer: 10 to 40 weight % PA1 (D) ethylenic unsaturated acid: 0 to 10 weight % PA1 (E) other copolymerizable monomer(s): 0 to 30 weight %
to a polymerization with one or more radical initiators.
Examples of alkyl(meth)acrylates which constitute the main monomer component of the above acrylic resin are C.sub.1 to C.sub.12 alkyl esters of (meth)acrylic acid, including methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, lauryl methacrylate, 2-ethylhexyl methacrylate and the like. Such monomer(s) is (are) used in an amount corresponding to 30 to 88 weight % of the total weight of monomers and chain transfer agent. As the second monomer, one or more hydroxy bearing ethylenically unsaturated monomers are used in an amount corresponding to 10 to 40 weight % of the total weight of monomers and chain transfer agent. Examples of such monomers are hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, diethylene glycol acrylate, diethylene glycol methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, and the members represented by either one of the following formulae: ##STR1## and the like.
In the above formulas, R is a hydrogen or methyl group.
If the amount of hydroxy bearing ethylenically unsaturated monomer is less than the abovesaid lower limit of 10 weight %, this results in a coating of insufficient strength and if it is over the upper limit of 40 weight %, this results in a resin composition having an increased viscosity and which is excessively crosslinked.
The monomers may further include ethylenic unsaturated acid in an amount of 0 to 10 weight % of the total weight of monomers and a chain transfer agent, but too excessive an amount of the ethylenic unsaturated acid is not recommended because of an undesirable lowering of the water resistance of the formed film.
Examples of the aforementioned ethylenic unsaturated acids are acrylic acid and methacrylic acid. The monomers may additionally include other copolymerizable monomers in an amount of 0 to 30 weight % of the total weight of monomers and a chain transfer agent. Examples of such monomers are styrene and styrene derivatives (e.g. t-butyl styrene, .alpha.-methyl styrene, vinyl toluene); nitrogen containing monomers, such as dimethyl aminoethyl acrylate, dimethyl aminoethyl methacrylate, diethyl aminoethyl acrylate, diethyl aminoethyl methacrylate, vinyl pyridine, acrylamide, N-methylol acrylamide, N-butoxyl methyl acrylamide, acrylonitrile; fumaric ester, itaconic ester, maleic ester and the like. As the most characteristic feature of the present invention, the abovesaid monomers are used in combination with 2,4-diphenyl-4-methyl-1-pentene as a chain transfer agent.
In the present invention, 2,4-diphenyl-4-methyl-1-pentene is specifically selected not only as an active chain transfer agent, but also as a resin constituting unit to improve the film properties as gloss, sharpness, water resistance and the like.
In contrast to the known findings in connection with the polymerization of vinyl monomers, in the present acrylic monomer polymerization system, the 2,4-diphenyl-4-methyl-1-pentene chain transfer agent has to be used in a larger quantity, i.e. in a range of 2 to 60 weight % of the total of the monomers and the chain transfer agent. More preferably, the chain transfer agent should be used in an amount corresponding to 5 to 40 weight % of the total of the monomers and the chain transfer agent.
If the said amount of chain transfer agent is less than 2 weight %, one cannot obtain an acrylic resin having a number average molecular weight of 500 to 5000 and the thus obtained resin cannot be used for the preparation of a high solids paint. Similarly, if the chain transfer agent is less than 2 weight %, the properties of the formed coating are not of satisfactory in respect to gloss, sharpness, water resistance and the like. On the other hand, if the said amount of chain transfer agent is over the upper limit of 60 weight %, this results in a coating with inferior film properties.
The present acrylic resin composition for high solids paint use is obtained by subjecting a mixture of the abovesaid monomers and the chain transfer agent, each in specified amounts, to a polymerization in the presence or absence of solvent and with one or more radical polymerization initiators. When used, the solvent may be selected from any of the known solvents customarily used in the preparation of an acrylic polymer, including xylene, n-butanol, isobutanol, Solvesso (trade mark), butyl acetate, isopropanol, dioxane and the like.
The radical polymerization initiator is also of common type and examples of suitable initiators are benzoyl peroxide, lauroyl peroxide, t-butyl hydroperoxide, acetyl cyclohexane sulfonyl peroxide, diisobutyl peroxide, t-butyl peroxy benzoate, diisopropyl peroxy dicarbonate, azo-bis-(2-methyl propionitrile), t-butyl peroxy-2-ethyl hexanoate, azo-bis-isobutyronitrile and the like.
The polymerization may be effected in a conventional way to obtain an acrylic resin having a number average molecular weight of 500 to 5000, preferably 800 to 4000 and more preferably 1000 to 2500.
If the number average molecular weight of the acrylic resin is less than 500, this results in a coating with inferior properties, whereas if it is over the upper limit of 5000, one is unable to formulate a high solids paint.
The acrylic resin of the present inventions is therefor quite useful as a resinous vehicle for a high solids coating composition capable of producing a coating with excellent film properties in respect to, inter alia, water resistance, weather resistance, coatings which have an anti-cracking appearance and the like. The present resinous composition is especially useful for the preparation of a curing type high solids paint by combining it with a hardner such as a melamine-formaldehyde resin, a urea-formaldehyde resin, a polyisocyanate and the like.
When a melamine-formaldehyde resin is selected, the weight ratio of the acrylic resin of the present invention to the melamine-formaldehyde resin is preferably determined in a range of 70/30 to 30/70. This is because if the amount of melamine-formaldehyde resin is less than the lower limit of 70/30, the cured coating is deficient in strength and if it is over the upper limit of 30/70, this results in an excessively hard and brittle coating.
As to the molecular weight of such melamine-formaldehyde resin (in the case when two and more melamine formaldehyde resins are used as, an average value), it is preferably determined to have a molecular weight limit of less than 1000. This is because, if it is more than 1000, a high solids paint with the desired properties can be hardly obtained with the present acrylic resin.
In the case when a polyisocyanate is selected as a hardner, it should be used in an amount capable of resulting in an equivalent ratio of NCO of the polyisocyanate the OH of the acrylic resin of 0.5 to 2.0, preferably 0.8 to 1.2. This is because, if the ratio of NCO/OH is less than 0.5, the cured coating is deficient in strength and if it is more than 2.0, no additional effect can be expected therefrom.
By the adoption of the above conditions, a highly desired high solids paint can be prepared, which is excellent in water resistance, weather resistance, appearance and the like. In preparing such paint, it is of course possible to add other additives such as a viscosity modifier (e.g. organic montmorillonite, cellulose acetate butyrate), a surface conditioner (e.g. silicone, organic high polymer), a UV absorber, a hindered amine, a hindered phenol, coloring matter and the like.
In the case of preparing a base coat composition for a 2-coat-1-bake metallic paint, metallic pigments such as aluminium flakes, copper bronze flakes and the like may be used with other optional coloring matter.
The inventor has also found that crosslinked polymer microparticles may be advantageously added to the high solids paint to further improve the sagging properties of the coating composition and the appearance of the 2-coat-1-bake type metallic paint.